It depends on the type of fabric you use (price per yard) and how much the cost of labor would be... Not a good answer, sorry.
Fabric can run you $20/yard and up. I picked one recently that was $65/yard because of its durability and other fabric qualities. The labor to recover all four cushions was around $400.
Be sure to find a good upholsterer. They are not all created equal.
Barry & Andrea
2014 Airstream Limited 30' ~ Silversled
2019 Chevy HD2500, 4X4, Crew Cab